It's inevitable your brain will work slower and slower as you get older — but leafy greens could put the brakes on that decline, according to a study published in the journal Neurology.

"Adding a daily serving of green leafy vegetables to your diet may be a simple way to help promote brain health,” said the study's lead author Martha Clare Morris, a professor from Rush University, in a statement.

Morris's study pored over data taken from nearly 1,000 US adults in their 80s, who documented what they ate (in particular, how often they ate the likes of spinach, lettuce, kale and other greens) and underwent cognitive assessments over a 10-year period.

Those who ate the most leafy greens showed a significantly slower rate of decline on tests of memory and thinking skills than those who ate the least — equivalent to their being 11 years younger in age, Morris calculated.

She said the results of the study are important because the number of people with dementia is rising, so factors that could reverse that trend need to be closely examined.

However, she cautioned that her study only shows a link between consumption of leafy greens and a reduced rate of cognitive decline, and not that the former directly causes the latter.

The Australian Dietary Guidelines define a serve of leafy greens as 75g — which works out to either half a cup if they're cooked, or a cup if they're eaten uncooked.